Engine retarders of the compression release type, also known as engine compression braking systems, are well known in the art. Such systems are designed to convert, temporarily, an internal combustion engine into an air compressor so that a retarding horsepower or braking action is established in the vehicle drive train. The basic design for an engine retarding system of the type referred to is disclosed in U.S. Pat. No. 3,220,392 assigned to Cummins Engine Company of Columbus Ind. In that design, a hydraulic system is employed wherein the motion of a master piston actuated by an appropriate intake, exhaust or fuel injector pushtube or rocker arm controls the motion of a slave piston which opens the exhaust valve of the internal combustion engine near the end of the compression stroke, whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke, but, instead, is dissipated through the exhaust and cooling systems of the engine.
In most Diesel engines, a mechanical fuel injector for each cylinder is driven from a third cam lobe of the engine cam shaft. It is therefore desirable to derive the motion for the compression release retarder from the fuel injector pushtube for the cylinder experiencing the compression release event. The fuel injector pushtube is a desirable source of motion both because it peaks very shortly after top dead center (TDC) position of the piston following the compression stroke and also because the effective stroke of the injector pushtube is completed in a relatively short period, e.g. 20-40 crank angle degrees.
The need for a compression brake master cylinder which opens the exhaust valves in a rapid fashion is discussed in U.S. Pat. No. 4,706,624 to Meistrick et al. The process and apparatus disclosed therein are directed towards cyclically storing energy in a plenum, releasing the stored energy from the plenum at a predetermined point in the travel of a master piston driven by an exhaust or fuel injector pushtube and directing the stored energy to a slave piston whereby the exhaust valve is opened rapidly at a predetermined time. Such an approach is referred to as indirect actuation or displacement of the exhaust valves since a hydraulic device stores and releases energy to move the valves.
Quenneville, U.S. Pat. No. 5,000,145, discloses a compression release retarding system wherein a master cylinder assembly includes a master piston of variable length. The variable length master piston travels a fixed distance to the pressure release point so that the timing of the compression release is precisely controlled and independent of installation and engine component tolerances. Rather than a direct displacement of a slave piston for a cylinder with a master piston, Quenneville teaches indirectly displacing the slave piston by a master piston which supplies high pressure hydraulic fluid to an accumulator and triggers release of the accumulated hydraulic fluid to the slave piston at an appropriate time as in the Meistrick et al. '624 patent.
A more simplistic master cylinder with a variable length telescoping piston which directly actuates a slave piston to open the exhaust valves in an internal combustion engine would enhance the operation of an engine compression braking system as well as provide a simplified approach to rapid actuation or opening of the exhaust valves of a particular cylinder in conjunction with limited excursion or displacement of the valves.